The present invention relates to magnetic bearings and particularly to active magnetic bearings having a very low "friction" couple.
An "active" magnetic bearing is made up of at least: a displacement detector, an electronic control unit, and an electromagnet. Any displacement sensed by the detector is counteracted by a servo-system which sets up opposing magnetic forces by means of the electromagnet. An active magnetic bearing is disclosed in, inter alia, U.S. Pat. No. 3,787,100. Such magnetic bearings generally incorporate a stator formed by an electromagnetic circuit having at least one pair of north and south poles. A rotor is mounted in the said magnetic bearing and the lines of force in the magnetic circuit induced in the stator close in a magnetic part of the rotor, which part is situated opposite the said stator.
However, although such magnetic bearings confer very great advantages whenever it is desired to have such a rotor turn at an extremely high speed, due to the fact that they allow any possibility of mechanical friction to be eliminated as a result of the absence of contact between the rotor and the stator, nevertheless they are still subject to various physical phenomena which, although not of mechanical origin, still have a retarding effect on the rotation of the rotor and for this reason will be referred to by analogy as friction.
This friction, which may detract greatly from the satisfactory operation of the magnetic bearing, has two main physical origins: on the one hand, losses due to eddy currents induced in the rotor and on the other hand hysteresis-effect losses due to the change in the magnetic field induced in the rotor and the stator. Eddy current losses can be reduced in a known way by using thin metal plates having high resistivity.
It is therefore an object of the invention to provide a magnetic bearing of the type which has a stator formed by an electromagnetic circuit having at least one pair of north and south poles, in which bearing the couples produced by hysteresis-effect friction are reduced to a minimum or are very appreciably reduced in comparison with those which exist in known magnetic bearings since they may be reduced by a factor of the order of 10 or even more.